Equipping wireless stations with multiple antennas is one of the widely adopted methods to increase the channel capacity of a wireless communications network. The multiple antennas on a transmitting and a receiving wireless station enable the two wireless stations to establish a multiple-input-multiple-output (MIMO) channel.
Various techniques, such as beamforming, space-time coding (STC), and cyclic delay diversity (CDD), have been developed to utilize MIMO channels. In order to establish a beamformed MIMO channel, a transmitting wireless station must acquire accurate channel state information about a destination wireless station. However, even without obtaining accurate channel state information, STC and CDD can fully utilize the multiple antennas on a wireless station to increase diversity of a MIMO channel.
All the MIMO techniques described above have limitations. When STC is applied to an antenna array of more than two antennas, the symbol rate is reduced. In addition, CDD may fail to achieve transmit diversity if cyclic delays are not selected properly. Specifically, the desired performance of a CDD transmission scheme (i.e., high transmit diversity and high symbol rate) depends on the selection of cyclic delays and channel estimation when there are limited pilot signals available.
A beamforming technique becomes ineffective when channel state information is not available or when a broadcast beamformed MIMO channel is created. A wireless communications network using beamforming may have a lower utilization rate because the acquisition of channel state information consumes channel resources and thus reduces the available channel capacity. For example, a transmitting wireless station can obtain downlink channel information based on channel information carried in uplink sounding signals and/or uplink traffic. The overhead resulting from the transmission of the uplink sounding signals limits the number of wireless stations that a wireless communications network can support. Because the channel condition between a mobile station and a base transceiver station is unique. It is almost impossible to obtain suitable beamforming weighting vectors to create a broadcast beamformed MIMO channel.
To deal with the limitations associated with individual MIMO techniques, a wireless communications network may utilize a variety of MIMO techniques to fully utilize the multiple antennas on a wireless station. Incorporating different MIMO techniques can improve the performance of a wireless communications network with or without channel information.